In general, chemicals exhibit absorption spectra that uniquely identify each chemical. This enables the concentration of a particular substance (referred to herein as the "test substance") within a sample to be measured by passing a beam of light through the sample and measuring the absorbance of the sample at one or more wavelengths characteristic of that substance. For example, CO.sub.2 is fairly unique in having an absorption peak at 4.26 microns, so that light of this wavelength can be used to detect the concentration of CO.sub.2.
Because Beer's Law applies in general only to monochromatic or nearly monochromatic light, the absorption by the test substance accurately reflects the concentration of the test substance only if the amount of absorption at each measured peak is separately determined. This can be achieved by use of a monochromatic source, such as a mercury vapor lamp, but this necessitates having available an optical source that produces light only at a wavelength that coincides with an absorption peak of the test substance. A suitable source will not generally be available that has an emission peak substantially coinciding with an absorption peak of the test substance. It is therefore common to utilize a broadband light source in conjunction with some sort of wavelength selective device to direct through the sample only light of the desired wavelength. This can be achieved, for example, by means of specially designed filters (for example, interference filters) that pass only the wavelength of interest. It can also be achieved by means of a diffraction grating that directs different wavelengths of light into different directions. An opaque barrier having an aperture is positioned to pass through the sample only light of the wavelength of interest.
Because of the large number of spectral absorption peaks of organic compounds in the infrared spectral range, it is important to provide optical sources that exhibit a strong intensity in the infrared range. This can be achieved by use of a blackbody radiation source, because such sources can be easily heated to a temperature that provides high intensity light within this wavelength region. Because the blackbody spectral distribution and intensity are uniquely determined by the temperature of the source of this blackbody radiation, the peak intensity can be adjusted by varying the temperature of the blackbody. By Wien's Displacement Law, the peak intensity of a blackbody radiation source of temperature T is at a wavelength .lambda..sub.MAX equal to 2.898.multidot.10.sup.-3 /T, where T is measured in degrees Kelvin and .lambda..sub.MAX is measured in meters. By proper selection of T and the size of the blackbody radiator, a desired intensity of blackbody light at a selected wavelength .lambda. can be achieved.
The spectrum from hot filament approximates blackbody radiation. Because the filament does not have a unit emittance over all wavelengths and because there is some temperature variation within the filament, it is not an ideal blackbody, but its emission spectrum still approximates a true blackbody spectrum over most of its spectral range. Thus, optical sources such as a hot filament lamp, will be referred to herein as a "quasi-blackbody" source. Unfortunately, because such lamps are typically enclosed by a quartz bulb and quartz is substantially opaque for wavelengths longer than about 4.5 microns, the optical spectrum of such optical sources is severely attenuated above 4.5 microns. Because the range of wavelengths for infrared light is from about 1 micron to about 1 mm, a hot filament lamp is not a good source of light for most infrared wavelengths.
Another type of blackbody light source is the Nernst Glower, which consists of a heating element, such as a tungsten filament, embedded in a ceramic block. Because of the large heat capacity of such a device, its peak wavelength and the intensity of its emitted light at a given wavelength cannot be quickly altered. In spectrometry, it is advantageous to be able to change the intensity and/or the wavelength of the optical beam quickly in order to change to a different peak wavelength or to modulate the light intensity. For example, a series of measurements can be performed more quickly if the light can be quickly turned on and off. Also, for the purpose of improving the signal-to-noise ratio, it is useful to be able to modulate the absorption signal, so that it can be separated from the background noise, which does not vary in intensity at this modulation frequency. Thus, it would be advantageous to have a blackbody light source that does not have an artificial cutoff (such as the hot filament lamp) and that enables the intensity and wavelength of the emission peak to be varied much more quickly than the Nernst Glower.